Optical modulators are used in a variety of optical systems to alter the intensity of a light beam, usually in a rapid manner that imparts information onto a carrier optical signal. In optical systems based on waveguides, such as optical telecommunication systems that employ optical fibers, optical modulators are used to encode information onto light guided by the waveguides. Other applications for optical modulators include their use as fast switches to perform logic operations, and to act as light gates that selectively block or transmit light without necessarily imparting information onto the light beam.
State-of-the-art optical modulators are typically either electro-optic-(EO)-based or electroabsorption-(EA)-based. The former are based on the linear electro-optic (Pockels) effect, while the latter exploit either the Franz-Keldysh effect or the quantum confined Stark effect in multiple quantum well heterostructures. EO-based modulators typically employ a Mach-Zehnder interferometer (MZI) fabricated in an EO material (usually, lithium niobate) using standard methods of fabricating integrated optical devices and systems (e.g., Ti-indiffusion, proton exchange, etc.). Unfortunately, since the MZI is external to the optical fiber, coupling between the MZI and the waveguide causes undesired optical signal attenuation. Also, the attenuation process involves dividing up a light beam into two light beams, imparting relative phases to the light beams, and then interfering the light beams. This makes for a rather complicated system and process.
EA-based modulators, on the contrary, can be integrated with a laser on a single chip. However, they typically have a relatively low extinction ratio (i.e., the ratio of power levels corresponding to logical “1” and “0”) of ˜10 dB. Currently used EAMs are bulky and their integration with systems and/or devices that have different materials is not straightforward.
Certain fiber optic modulators seek to rely on the use of surface plasmons. A surface plasmon (SP) is an electromagnetic wave that travels at the interface between a metal layer and a dielectric layer (which can be air) when certain conditions are met. SP-based optical modulators utilize a structure that selectively converts the power carried by a guided light wave into the SP. By selectively varying the amount of power converted to the SP, the guided light wave is selectively attenuated to achieve the required analog or digital modulation.
To date, SP-based fiber optic modulators rely on lateral (i.e. parallel to the fiber axis) light coupling to the particular metal-dielectric structure in order to achieve the necessary conditions for exciting the SP. For example, light intensity modulation is achieved in one instance by evanescent coupling of the propagating mode of the waveguide and the plasmon mode supported by a metal-dielectric interface external to the waveguide. Other approaches use prisms or grating to obtain an oblique incident angle to excite the SP.